1. Field of the Invention
The present invention relates to a circuit generating a stable reference voltage with respect to temperature.
The invention relates, particularly but not exclusively, to a circuit generating a stable reference voltage with respect to temperature for CMOS process, the detailed description that follows covering this field of application for convenience of explanation only.
2. Description of the Related Art
As is well known, a requirement of most integrated electronic circuits is that at least one reference voltage be generated internally of the semiconductor chip in which they have been integrated.
An example of reference voltages internally generated a chip are VCM reference voltages having levels intermediate between the supply voltage values and being intended for use by several circuit sections integrated in the chip.
In particular, these internally generated reference voltages should be stable with respect to temperature and be unaffected by possible variations in the supply voltages, such as variations caused by rippling on the supply lines.
To provide a voltage reference that be unaffected by ripple, an internal reference voltage is normally used which is already provided in the chip structure.
In a typical P-substrate CMOS process, this internal reference voltage would be the base-emitter voltage Vbe of a parasitic PNP bipolar transistor created in the integrated circuit during the process.
This voltage actually exhibits a degree of dependence on temperature that can be eliminated, at least as concerns its first order component, by adding a compensating voltage to it, the latter voltage being quite easily obtained across a resistor through which an appropriate current flows.
More particularly, since the base-emitter voltage Vbe has a negative temperature coefficient, the compensating voltage is adjusted to have a positive temperature coefficient.
A simple known type of circuit adapted to generate such a reference voltage, compensated for temperature variations, is that shown generally and schematically at 1 in FIG. 1.
In particular, the generator circuit 1 comprises a bipolar transistor T1, specifically of PNP type, which is connected between a first voltage reference, e.g., a supply voltage Vcc, and a second voltage reference, e.g., a ground reference GND. In the instance of the integrated circuit that contains the voltage generator being formed with CMOS technology, a parasitic transistor may be utilized as transistor T1.
The bipolar transistor T1 has a first conduction terminal, which may be the collector terminal, connected to ground GND; a second conduction terminal, which may be the emitter terminal, connected to an internal circuit node X1; and a control terminal, i.e., the base terminal, connected to the first conduction terminal and ground.
This internal circuit node X1 is connected to the supply voltage reference Vcc through a series of a resistive element R1 and a generator G1 generating a current I1. The reference voltage Vref sought is picked off an output terminal OUT1 of the generator circuit 1, between the resistive element R1 and the generator G1.
To compensate for the thermal dependence of the base-emitter voltage Vbe of the bipolar transistor T1, the generator G1 supplies a current I1 having a positive coefficient of dependence on temperature.
As the skilled persons in the art know well, such a current value may be obtained by making use of a pair of parasitic bipolar transistors, biased to different current densities, from which a base-emitter voltage difference xcex94Vbe is derived for application to a resistive element of resistance R, so as to obtain a current:
I1=xcex94Vbe/R.
The current I1 thus obtained has a positive coefficient of dependence on temperature, and the reference voltage Vref at the output terminal OUT1 is, therefore, compensated for temperature.
It should be noted that in most cases, a current having these characteristics would be already provided in analog integrated circuits of the CMOS type, where it is used for biasing operational amplifiers, for example.
However, in such circuits, the reference voltage Vref, also known as the band-gap voltage, has in practice to approach the band-gap voltage of the silicon layer in which the whole circuitry is formed, in order to achieve good compensation of the temperature coefficients. This voltage is a physical constant that depends on the type of semiconductor employed, it being approximately 1.2 V for silicon.
Thus, the generator circuit 1 of FIG. 1 cannot provide reference voltages Vref that arestable with respect to temperature but displaced from the value of the band-gap voltage (1.2 V) for silicon. It is sometimes necessary, however, to have temperature-stable reference voltages generated which lie far from this value.
A prior approach to meeting this requirement is shown schematically in FIG. 2.
In particular, FIG. 2 shows a circuit 2 generating a stable voltage with respect to temperature, which circuit comprises essentially an operational amplifier OA2 having a first non-inverting (+) input terminal connected to a band-gap circuit BG2 adapted to supply the operational amplifier OA2 with a stable reference voltage with respect to temperature, Vref of about 1.2 V, same as in the prior approach just described.
The operational amplifier OA2 is in a buffer configuration having a first resistive element R21 connected between an output terminal and an inverting (xe2x88x92) input terminal of the amplifier OA2, and a second resistive element R22 connected between the inverting (xe2x88x92) input terminal and a voltage reference, e.g. a ground reference GND.
The generator circuit 2 uses the operational amplifier OA2 to convert the resulting stable voltage Vref=1.2 V provided by the band-gap circuit BG2 into another voltage KVref, where K is the gain of the buffer comprising the operational amplifier OA2.
In this way, any stable voltage value other than the band-gap value (equal approximately to 1.2 V) of the silicon layer can be derived from the temperature-stable voltage Vref.
To achieve values of the coefficient K greater than 1, the operational amplifier OA2 must be used in the non-inverting configuration.
While on several counts advantageous, there are drawbacks to this approach, among which:
an operational amplifier OA2 must be added to the chip own circuitry, resulting in more chip area and power being used up; and
large resistors R21 and R22 must be used in order to limit power consumption by the generator circuit 2, resulting in further expenditure of chip area.
A further prior approach is based on the observation that many analog integrated circuits, especially those provided with converters, include differential circuits adapted to provide two voltage values whose difference xcex94V is stable with respect to temperature. A circuit that provides a temperature-stable voltage that is a different value from the silicon layer band-gap value, based on the voltage difference xcex94V, is shown generally and schematically at 3 in FIG. 3.
This circuit 3 comprises essentially an operational amplifier OA3, having an inverting (xe2x88x92) input terminal connected to a first input terminal IN31 of the circuit 3 through a first resistive element R31, and having a non-inverting (+) input terminal connected to a second input terminal IN32 of the circuit 3.
In particular, a voltage difference xcex94V is established between the input terminals IN31 and IN32, which is stable with respect to temperature.
The operational amplifier OA3 also has an output terminal connected to the respective control terminals of first and second MOS transistors M31 and M32.
The first transistor M31 is connected between the inverting (xe2x88x92) input terminal of the operational amplifier OA3 and a ground reference GND, while the second transistor M32 is connected between a current-mirror circuit CM and ground GND.
This current-mirror circuit CM is also connected to an output terminal OUT3 of the circuit 3, and connected to ground GND through a third resistive element R33.
A voltage value Vout3 is obtained at the output terminal OUT3 of the circuit 3 which may have any selected value and is stable with respect to temperature, based on a voltage difference xcex94V that is also stable with respect to temperature. This is achieved by matching the resistive elements.
The operation of this prior circuit 3 will now be described. The temperature-stabilized voltage difference xcex94V is converted into a current I3=xcex94V/R31 that flows through the first transistor M31. The feedback loop from the first transistor M1 to the inverting (xe2x88x92) input terminal of the operational amplifier OA3 pulls the output terminal of the operational amplifier OA3 to a voltage level adequate to force the first transistor M31 to invite a current equal to xcex94V/R31.
This current I3 is suitably mirrored by the current-mirror circuit CM onto the second resistive element R32, and generates an output voltage Vout at the output terminal OUT3 of the circuit 3, which output voltage will be Vout=nxcex94V(R32/R31), where n is a proportional value to the mirroring ratio of the current-mirror circuit CM, comprising the aspect ratios of the transistors M31 and M32.
While achieving its objective, not even this approach is destitute of shortcomings.
In particular, the circuit 3 still employs an operational amplifier OA3, involving added use of integration area and power consumption.
Furthermore, the output impedance of the circuit 3 equals the resistance of the second resistive element R32. This resistance cannot be too low, in order to avoid a large waste of the current drain of circuit 3 to obtain the voltage Vout sought.
For this reason, an additional buffer circuit often has to be introduced after the structure of FIG. 3.
Thus, the circuit 3 is a seldom-used solution.
An embodiment of the invention is directed to a circuit generating a reference voltage that is stable with respect to temperature and has structural and functional features appropriate to overcome the drawbacks that beset the circuits according to the prior art.
The reference voltage generating circuit has a feedback resistive element connected to a bipolar transistor used for generating a desired reference voltage, the feedback resistive element being operative to supply the bipolar transistor with a suitable current value effectively self-compensating for the dependence on temperature of the transistor base-emitter voltage.
The reference voltage generating circuit is connected between first and second voltage references, and comprises at least one current generating circuit adapted to inject a reference current into a resistive element that is connected between a base terminal of a bipolar transistor and an additional voltage reference, said bipolar transistor being connected between said first and second voltage references and connected to an output terminal of said generator circuit, whereat said stable reference voltage with respect to temperature is present, and further comprising at least another resistive element, feedback connected between said output terminal of said generator circuit and said base terminal of said bipolar transistor to enable injecting additional current, having reverse dependence on temperature from said reference current, into said resistive element.
The features and advantages of the circuit generating a stable reference voltage with respect to temperature, according to embodiments of the invention, will be apparent from the following detailed description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings.